Localization, Purification, and Characterization of Shikimate Oxidoreductase-Dehydroquinate Hydrolyase from Stroma of Spinach Chloroplasts

The stroma of chloroplasts is probably the sole site of the shikimate pathway enzymes shikimate oxidoreductase/dehydroquinate hydrolyase (SORase/DHQase) in spinach leaves. (a) The chromatographic behavior of the bifunctional protein SORase/DHQase on several separation materials with extracts from stroma compared with leaf extracts showed only one peak of enzymic activity originating from the stroma. (b) Polyacrylamide gel electrophoresis (PAGE) of these extracts followed by specific staining resulted in the same pattern without a band of extraplastidic enzyme. (c) In protoplast fractionation experiments it was shown that SORase/DHQase was present only in the soluble chloroplast protein fraction. An improved purification procedure for SORase/DHQase from stroma of chloroplasts, yield 40%, 1600 times as pure, gave essentially one protein band on sodium dodecyl sulfate-PAGE. Our results demonstrate that both enzyme functions are carried out by a single polypeptide. Nondenaturing PAGE exhibited a pattern of four bands with SORase/DHQase showing that they differ in charge but not in their molecular weight. Molecular weight was determined to be 67 kilodaltons (gel filtration) and 59 kilodaltons (PAGE) for all four forms. It was proven they were not due to artifacts. The four forms show similar kinetic properties, their Km and pH optima differing only very slightly. Response to some metabolites is reported.